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The treatments are randomly assigned to all of the experimental units in a completely randomized design.

In a double-blind experiment, neither the subjects nor the persons who interact with them and evaluate their responses are aware of the therapy they received.

Observational research looks at people and assesses factors of interest without trying to affect their responses.

Individuals are purposely subjected to a treatment in order to measure their responses in an experiment.

According to the question, a researcher looked into a group of identical twins who had been separated and adopted at infancy. As a result, we'll calculate the mean for both twins as follows: $$\begin{gathered} \overline{x_A}=\frac{120+99+....+113+100}{12} \\ =103.6667 \end{gathered}$$

$$\begin{gathered} {\overline{x}}_B=\frac{128+104+....+114+112}{12} \\ =109.5 \end{gathered}$$

The difference in IQ scores is around five points, however since the sample size is only $12$, this variation could be attributable to chance. As a result, we can infer that there is no statistically significant evidence that identical twins from low-income families have lower IQs later in life than their high-income counterparts.